FitUp

Guide · Fittings and cut length

How to figure take-outs and cut length

Short answer

Cut length = center-to-center length − the take-out of each fitting − the root gap at each butt weld. For a two-elbow offset with butt welds that is travel − 2 × take-out − 2 × gap. A 10 inch offset with 2 inch 45 LR elbows and 1/8 inch gaps: 14.142 − 2 × 1.375 − 2 × 0.125 = 11.142, marked 11 1/8.

Layout dimensions are taken to the centers of fittings, because that is where the lines on a drawing meet. Pipe does not reach those centers. It stops at the end of each fitting, and at a butt weld it stops a gap short of that. The cut length is what remains once each of those has been taken away.

A 90 degree elbow: the take-out, or center-to-end, is the distance from the point where the two pipe centerlines cross to the end of the fitting take-out (center to end) take-out center
Your layout measures center to center, but the pipe stops at the end of the fitting. Each fitting takes out its center-to-end from the length you cut.

What to subtract, by joint type

Butt-weld fittings

Subtract the fitting's center-to-end, from the maker's chart or a take-out chart, plus one root gap per joint. The gap comes from your welding procedure; 1/16 to 1/8 inch is common for pipe, and FitUp uses 1/8 by default for welded trades.

Threaded fittings

The pipe screws into the fitting, so part of it is inside. Subtract the center-to-end less the thread engagement, the length of pipe that goes into the fitting. Engagement changes with pipe size and with how far the joint is made up, so take it from the fitting maker or measure a made-up joint. There is no weld gap.

Socket-weld fittings

The pipe slides into a socket and is pulled back off the bottom before welding to leave an expansion gap, about 1/16 inch under most procedures. Subtract the center-to-socket-bottom dimension plus that pull-back gap. There is no root gap to add on top.

Steps

  1. Get the center-to-center length

    For an offset, that is the travel from the offset formula. For a straight piece, it is the dimension on the drawing.

  2. List the fittings at each end

    Size and type for each end separately. They are not always the same.

  3. Subtract each fitting's allowance

    Center-to-end for butt-weld, center-to-end less engagement for threaded, center-to-socket-bottom plus the pull-back for socket-weld.

  4. Subtract a root gap for each butt weld

    Two butt-welded ends, two gaps.

  5. Round the result once

    Keep decimals until the end, then mark to the nearest 1/16.

Worked examples

A 45 degree offset, butt-welded

Travel = 10 × 1.41421 = 14.1421 Take-outs = 2 × 1.375 (2 in 45 LR) = 2.75 Gaps = 2 × 0.125 = 0.25 Cut = 14.1421 − 2.75 − 0.25 = 11.1421, marked 11 1/8

A straight piece between two 90s

36 inches center to center between two 3 inch 90 LR butt-weld elbows (4 1/2 inch center-to-end each), 1/8 inch gaps:

Cut = 36 − 2 × 4.5 − 2 × 0.125 = 26.75, marked 26 3/4

Nine and a quarter inches of that 36 is fittings and gaps. On short spools the allowances can be most of the dimension.

Why offsets come up long

When a piece is long at fit-up, it is almost always one of these:

  • One take-out instead of two. There is a fitting at each end of the piece.
  • The gaps forgotten. Two butt welds at 1/8 inch is 1/4 inch, and a spool with six joints loses 3/4 inch to gaps alone.
  • The chart, not the fitting. Center-to-end varies between makers. Measure a fitting from the cart when the job is tight.
  • A factory 45's take-out used for a cut 90. A 90 cut down to 45 takes out R × tan 22.5°, which is not always the factory 45 value. See cutting down a 90.
  • Rounding every step. Rounding the travel and each take-out to sixteenths before subtracting can put the cut a sixteenth off. Round once.
  • Cutting the travel. The travel is the center-to-center number, the start of the arithmetic, not the end of it.

Related questions

Let the app keep track

FitUp lists the take-out and gap under every cut length, as "2 × 1 3/8" and "2 × 1/8", so you can see what came off before you mark the pipe. Its take-out chart can be corrected to your fittings with Pro, and every offset after that uses your values.

Verify measurements and fitting dimensions before cutting.